1. Field of the Invention
The present invention relates to powder detergent compositions that have improved solubility in the laundering solution. More particularly, it relates to the addition of an acidulant to improve the solubility of powder detergent compositions in the laundering solution.
2. Discussion of Related Art
Granular laundry detergents containing admixed sodium carbonate are known to exhibit poor solubility under certain conditions. This poor solubility can cause clumps of detergent, which appear as solid white masses remaining in the washing machine and on washed clothes. Such clumps usually occur when the detergent is placed in a pile in the washing machine, particularly during cold water washes and/or when the order of addition to the washing machine is laundry detergent first, clothes second, and water last. The clumps may also occur when the powdered detergent is trapped within the folds or pockets of the fabrics to-be-washed, particularly in machines that do not provide for adequate agitation. It is believed that one contributor to this solubility problem is caused by hydration of the sodium carbonate and/or particle bridging resulting in a poorly soluble mass before the granular detergent can be dispersed and solubilized in the laundering solution.
Another problem exists when the laundry detergent contains high levels of nonionic surfactant. When such a detergent is added to the wash water, particularly when the temperature of the wash water is cool, the nonionic surfactant does not immediately solubilize. Instead, the surfactant may tend to gel resulting in a sticky mass which may deposit on the fabric before sufficient wash water is present to solubilize the nonionic surfactant.
U.S. Pat. No. 5,300,250 to Morgan et al. discloses that the addition of low levels of hydrophobic amorphous silicate material to granular laundry detergents containing admixed sodium carbonate improves their solubility in the laundering solution and eliminates or reduces the problem of clumps remaining in the washing machine and on washed clothes. The hydrophobic amorphous silicate material acts as an anti-caking agent and flow aid. The detergent is prepared by spray drying aqueous crutcher mixes of the surfactant and additives together with a premix containing sodium carbonate and hydrophobic amorphous silicate material.
U.S. Pat. No. 5,338,476 to Pancheri et al. discloses that spray dried granular laundry detergents having admixed sodium carbonate can achieve improved solubility in the laundering solution by incorporating citric acid. They report that they believe that the citric acid rapidly reacts with the sodium carbonate in the laundering solution to release carbon dioxide and helps to disperse the detergent and minimize the formation of insoluble clumps. The use of citric acid, in this manner, however, may not be desirable because a substantial portion of the citric acid may become neutralized to sodium citrate during storage. It is believed that the citric acid, which is hydroscopic, will absorb the free water present in the powder detergent formulation as well as in the atmosphere and become neutralized. The neutralization causes an unwanted increase in detergent particle size, powder lumps in the box, and loss of the desired effervescent effect.
U.S. Pat. No. 5,002,758 to Ichii et al. discloses bubbling bathing preparations preferably in the form of a tablet that contain fumaric acid and a carbonate together with carboxymethyl cellulose or an alkali metal salt or polyethylene glycol and less than 0.1% of a nonionic surface active agent. They also disclose that other organic acids may be used, for example, citric, tartaric, malic, malonic, pyridone carboxylic, succinic, adipic, phosphoric, and their salts.
A particular problem arises with the use of high density laundry detergent powders, i.e., those with bulk densities of 650 g/l or greater. Denser powders such as those of 800 g/l or higher are even more problematic. While these powders provide consumers the benefit of concentration and lower dosages, the processes required to produce high densities leave little or no void space in the detergent powder. For example, U.S. Pat. No. 5,133,924 describes a process that reduces the intraparticle porosity so that the void space is substantially decreased. These highly concentrated powders, however, can prove difficult to dissolve since the powder has little or no free space to allow the entry of water necessary for dissolution. This, in turn, can result in the powder forming localized areas of gelation which remain undissolved at the end of the wash cycle and contribute to residue. As a result, they are more susceptible to the cold water clumping problems.
U.S. Pat. No. 5,415,806 to Pepe et al. describes high density laundry detergent compositions having a bulk density of 650 g/l or greater and intraparticle porosities of about 25% or less. They report that acceptable solubility and dispersion is achieved by including a C.sub.2-4 alkylene oxide condensation product as a solubility aid. The process of making the described detergent composition includes preparing a base powder by mixing water plus detergent components in a slurry and spray drying the slurry. Consequently, the described process does not offer an improvement to the known disadvantages of spray drying. In addition, the compositions are those with high density but low porosity. As a result, the amount of surfactant that can be effectively loaded is restricted. Moreover, without the solubility aid it is likely that the detergent would not be effectively dissolved or dispersed.